1. Field of the Invention
This invention relates to the investigation of the earths subsurface, and in particular to methods and devices that utilize electromagnetic waves produced by power line harmonics to facilitate detailed subsurface investigations.
2. Description of the Related Art
There are numerous methods used to investigate the subsurface of the earth. The most widely used methods include seismic, gravity, magnetic, electrical, and electromagnetic. Electromagnetic techniques can be broadly divided into two categories based on the source of the electromagnetic waves (Dobrin et al). Natural field sourced methods include magnetotellurics (MT) and audio frequency MT (AMT). Artificially controlled source methods include transient soundings (TEM), ground penetrating radar, and controlled source audio magnetotellurics (CSAMT). These methods all have resolution and operational characteristics, which result in limited applicability and/or high costs.
It is desirable to have methods and devices that can be used in areas where the existing methods or equipment are not suitable technically, economically or logistically. It is desirable to have methods and devices that can have wide applicability, and are simple and inexpensive to operate, and provide high resolution.